Modern laboratory and industrial microscopy systems incorporate digital imaging to achieve both high resolution and throughput. These systems rely on the provision of an automatic focus (autofocus) capability. The technical problems that arise in autofocus applications are defined in large part by the limits of the imaging system. By example, high magnification (e.g. 50.times.) microscopes may have a working-distance to depth-of-focus (DOF) ratio of 10,000, thereby rendering conventional autofocus search techniques laboriously slow and inaccurate. Furthermore, the optical properties associated with the image plane typically vary from image to image, making conventional amplitude-dependent autofocus evaluation techniques less than optimum.
It is thus one object of the invention to provide an autofocus technique that provides high accuracy, high speed to achieve greater throughput, a large capture range, and a reduced sensitivity to image plane optical property variations.